The most remarkable occurrence of chromium in nature is found in ruby and emerald gemstones, where replacing Al3+ by Cr3+ produces the characteristic red and green colors of the ruby and emerald gemstones, respectively. Chromium exists in oxidation states ranging from Cr2+—Cr6+, wherein Cr3+ is thermodynamically the most stable and also the most important, being an essential trace element with a daily uptake of approximately 25-50 μg. However, in the higher oxidation states, e.g., as Cr6+, it is extremely dangerous being 500-1000 times more toxic than Cr3+. Long term exposure to Cr6+ can lead to lung cancer, chronic bronchitis, asthma, emphysema, pulmonary fibrosis and other kinds of diseases (Reynolds et al., 2007; Zhitkovich, 2005; Levina and Lay, 2005). According to the US Department of Health and Human Services, the maximum exposure limit in air varies between 0.5-100 μg/m3, whereas in water the maximum exposure limit is 100 μg/l (ATSDR Chromium Toxicity, US Department of Health and Human Services, 2000).
The main sources for Cr6+ are modern chemical and industrial processes including oil and coal combustion, manufacturing of textile dyes, fabrication of nuclear weapons, chrome plating, metal finishing, and leather and wood preservation. These processes create a vast amount of toxic waste (Singh and Gupta, 2007; Liu et al., 2006; Gheju and Iovi, 2006; Mytych and Stasicka, 2004; Kieber et al., 2002). For example, the United States annually emits tons of Cr6+ as atmospheric pollution. Besides that, there is additional pollution of Cr6+ as waste water (ATSDR Chromium Toxicity, US Department of Health and Human Services, 2000). Therefore, selective detection and quantification, together with detoxification of Cr6+, are of high importance.
Current detoxification of Cr6+ is based on chemical reduction with stoichiometric amounts of iron(sulfur) salts followed by precipitation with a base (Eary and Rai, 1988; EPA, 1980, 2000). Although several sophisticated techniques are available to detect and quantify Cr6+ (Marqués et al., 2000), a selective and cost-effective sensor system with minimum requirements for sample preparation is highly desirable. Alternative approaches are rare (Boiadjiev et al., 2005; Tian et al., 2005; Turyan and Mandler, 1997; Ji et al., 2001). Cr6+ undergoes reduction in solution in the presence of H+ and low-valent metal centers such as Fe2+, Mn2+, V3+ or Os2+ (Espenson, 1970; Davies and Espenson, 1970; Birk, 1969; Westheimer, 1949). For example, [Os(bpy)3]Cl2 reacts with K2Cr2O7 in water under acidic conditions (pH=1) to afford Cr3+, as may be indicated by electron spin resonance (ESR) spectroscopy. Monolayer chemistry is rapidly developing (Collman et al., 2007; Yerushalmi et al., 2004; Liu et al., 2003; Lahann et al., 2003; Gupta and van der Boom, 2006, 2007; Gupta et al., 2006, 2007; Baker et al., 2006; Gulino et al., 2005; Basabe-Desmonts et al., 2004; Ashkenasy et al., 2000; Crooks and Ricco, 1998), and such, well-designed interfaces have been used to detect various analytes (Gupta and van der Boom, 2006, 2007; Gupta et al., 2006, 2007; Baker et al., 2006; Gulino et al., 2005; Basabe-Desmonts et al., 2004; Ashkenasy et al., 2000; Crooks and Ricco, 1998). However, the design of a suitable platform for detecting specific metal ions in a matrix remains a challenging task (Gupta and van der Boom, 2007; Zhang et al., 2006).
International Publication No. WO 2006085319 and the corresponding US Publication No. 20070258147, herewith incorporated by reference in their entirety as if fully disclosed herein, disclose a device having reversibly changeable and optically readable optical properties, the device comprising a substrate having an electrically conductive surface and carrying a redox-active layer structure configured to have at least one predetermined electronic property including at least one of electrodensity and oxidation state, said at least one electronic property being changeable by subjecting the layer structure to an electric field, wherein the electronic property of the layer structure defines an optical property of the structure thereby determining an optical response of the structure to certain incident light, the device enabling to effect a change in said electronic property that results in a detectable change in the optical response of the layer structure.
The aforesaid US 20070258147 further discloses a sensor device configured and operable for sensing at least one predetermined cation, anion, radical, liquid or gas substance, the device comprising a redox-active layer structure selected to be capable of changing its oxidation state in response to a reaction with said at least one substance, thereby causing a change in optical properties of said structure, said change being reversible and being optically readable. As defined in the aforesaid US publication, the cation to be recognized said sensor device may be selected from the group consisting of [Ru(phen)3]3+, [Ru(bipy)3]3+, [trianthrene]+, [Fe(bipy)3]3+, Pu4+, Au+, Ag2+, Ag+, Ce4+, Ru3+, Ir3+, Ir4+, Rh+, Rh2+, U2+, U3+, U4+, U5+, Rh3+, Pd2+, Pd4+, Pt2+, Pt4+, Ni2+, Ni4+, Co3+, Hg2+, Cu2+, Cu+, Cd2+, Pb2+, Pb4+, Sn2+, Sn4+, W+, NO+, Fe2+, Fe3+, an actinide and a lanthanide cation. In a particular embodiment, the redox-active layer structure of said sensor device comprises the osmium polypyridyl compound bis(2,2′-bipyridine)[4′-methyl-4-(2-(4-(3-propyltrimethoxysilane)pyridinium)ethenyl)-2,2′-bipyridine]osmium(II)[tris(hexafluorophosphate)/tri-iodide], respectively.